Using a mixing apparatus to suspend solid particles in a liquid is a common process step in many sectors of industry. Typically a mixing apparatus comprises a tank that is a cylinder having either a flat or a dished bottom. The mixing apparatus also includes an agitator having a shaft that extends downwards from the top part of the tank and an impeller mounted at the end of the shaft near the bottom of the tank. The impeller is usually positioned at the center of the tank above the bottom and its rotation creates a movement in the liquid being mixed. Depending on the application, either an axially or radially pumping impeller or a mixed flow impeller can be used. The mixing tank additionally contains baffles which are meant to direct the flow of liquid in the desired direction and reduce vortexing.
When designing a mixing apparatus for suspending solids in a liquid, several factors need to be taken into account. First, usually a uniform suspension is ideal, which means that the mixing should be effective enough to avoid the solids from accumulating at the bottom of the tank (typically this happens in the form of fillets circulating the rim of the tank bottom). On the other hand, the energy consumption of the process increases with speed of the impeller and the mixing effectiveness of the impeller blades. Additionally, faster impeller speeds increase shear forces in the liquid and thus potentially lead to unwanted particle breakdown in certain applications.
Although impeller design has attracted research interest in increasing the mixing efficiency, the effect of the tank bottom has gained significantly less attention. Chudacek, 1985 (Chudacek, M. W. Solids Suspension Behavior in Profiled Bottom and Flat Bottom Mixing Tanks, Chem. Eng. Sci., 40: 385-392) proposed a so-called a “cone and fillet” bottom design to improve suspension efficiency compared to the flat-bottom tank design. In the design suggested by Chudacek, the impeller is positioned above the center of a flat tank bottom and a right circular cone is positioned under it so that the axis of the rotation of the impeller and the apex of the cone are vertically aligned. An oblique fillet is arranged to run along the rim of the tank bottom.
Hydrometallurgy in general means the aqueous processing of metals. It is used to harvest metals from ores and is thus important for mining industry. Especially in the leaching stage, the ore in particulate form is mixed with a leaching solution to keep the particles suspended in the solution and to maximize the contact between the ore and the liquid for efficient metal extraction. The ore particles are typically heavy and require efficient agitation to avoid sedimentation. Currently, the mixing apparatuses used in demanding applications in hydrometallurgy have usually a dished-bottom, which functions more efficiently than a flat bottom, but is a rather expensive structure to build. Further, reduction in energy consumption and process optimization are continuous goals in mining industry to improve both the revenues for investment and environmental compatibility of the industry. The inventors have recognized the need for improving mixing efficiency while at the same time keeping energy consumption and strain on the devices as small as possible.